This invention relates to a system for separating pellets formed by subdivision of a parent semiconductor wafer and, more particularly, to an improved pellet separation system employing a "stretchable membrane."
In the very competitive and cost conscious semiconductor industry there is a constant search for ways to improve the efficiency of manufacturing processes and thereby to reduce costs. For example, much attention is currently being given to efforts to fabircate semiconductor devices from ever larger wafers.
Utilization of a larger wafer makes problems that are encountered during wafer subdivision more significant. In general, the two major problems associated with wafer subdivision include the difficulty of supporting the wafer during subdivision and the difficulty of supporting and handling the discrete pellets formed by subdivision. Prior improvements in the subdivision process have generally tended to alleviate only one of the problems and have had little or no effect on the other. In certain instances proposed methods for solving one problem have even had deleterious effects with respect to the second problem. An example of a system with deleterious side effects is the scribing and subdividing method in which a wafer is affixed to a metal shim by an adhesive wax. The wafer is scribed and the shim is flexed causing wafer fracture and subdivision. The system proved to be an excellent way to support a wafer during the subdivision process. However, handling of the individual pellets provided was difficult inasmuch as the wax adhered tightly to the pellets and thus the pellets were difficult to remove from the shim. Furthermore, a residual layer of wax generally remained on the pellets following their removal from the shim. This residual layer frequently had deleterious effects with respect to remaining processing steps.
Consequently, it was desired to eliminate the utilization of a layer of wax on the wafer. As a result, a method of "shimless scribing" was developed. This method of shimless scribing is fully described in U.S. Pat. No. 3,559,855, which is assigned to the assignee of the present application. Shimless scribing provides an excellent method for subdividing semiconductor wafers without requiring waxes or other potential contaminants. However, it is still difficult to handle the individual pellets following shimless scribing.
Another approach, taken by others, concentrated on alleviating handling problems following subdivision. In this method, a subdivided wafer is placed on a membrane which is then stretched in all directions to separate the pellets but maintain them in the same relative orientation. This stretchable membrane method is fully described in U.S. Pat. No. 3,448,510 (Bippus). A problem that has been found when utilizing the technique of Bippus is that the subdivision process itself is still very difficult and awkward. For Example, FIGS. 10A thru 10R of Bippus illustrate the process of subdividing a semiconductor wafer into pellets and separating the pellets for final delivery to an assembly area. Eighteen separate Figures are required to show the process. Only the last four Figures relate to the membrane stretch and subsequent steps. The first fourteen Figures illustrate the subdivision of the wafer. Consequently, it will be appreciated that while Bippus has taught an effective way of separating the pellets in a subdivided wafer once the fractured wafer has been placed on a stretchable membrane, realization of the full potential of his system will only be had when the subdivision process itself can be carried out more easily than as taught by him.
It is an object of this invention, therefore, to provide a system for subdividing semiconductor wafers into pellets that complements the stretchable membrane handling techniques thus permitting the realization of the full potential of the stretchable membrane system.